In article ,
Domenic wrote:

Assuming that your sheets are named Sheet1, Sheet2, etc., and that
B1:B10 contains your sheet names, and C1:C10 contains the corresponding
flag, Yes/No, try the following formula...

=SUM(SUMIF(INDIRECT("'Sheet"&SMALL(IF(C1:C10="Yes" ,ROW(C1:C10)),ROW(INDIR
ECT("1:"&COUNTIF(C1:C10,"Yes"))))&"'!A1"),"<"))

Hope this helps!

I forgot to mention that the formula must be confirmed with
CONTROL+SHIFT+ENTER, and not just ENTER.

Hope this helps!